B cell development generates a large and diverse repertoire of mono-specific and self-tolerant B cells expressing surface immunoglobulin (Ig) as the recognition component of their antigen receptor. Ig diversity depends upon a tightly regulated novel site-specific DNA recombination reaction known as V(D)J recombination which assembles the variable exons of the Ig heavy-chain and light-chain genes from their component gene-segments. A pair of lymphoid-specific proteins, RAG1 and RAG2, form a complex with one another that recognizes pairs of rearranging gene segments and helps catalyze their joining. This process is regulated during lymphoid development with respect to lineage (Ig genes fully rearrange in B but not T cells), order within the lineage (heavy-chain rearranges before light-chain), and the use of alleles (an individual cell expresses only one functional Ig molecule on its surface, a phenomenon known as allelic exclusion). Previous work has shown that targeting of the recombinase depends upon the accessibility of gene- segments within chromatin structure and that transcription of unrearranged gene segments correlates with their accessibility to the recombinase. This competing renewal of a long-standing program of research on the regulation of B cell development aims to understand the mechanisms which regulate V(D)J recombination during generation of the primary B cell repertoire through the pursuit of four specific aims. 1) To identify the cis-acting DNA sequences and trans-acting factors which are required for the transcriptional regulation of the RAG1 and RAG2 genes;2) To determine how the Ig kappa locus is activated for transcription and rearrangement in pre-B cells in such a fashion that only one allele is functionally rearranged;3) To determine what role if any the transcription factor NF-kB plays in the regulation of receptor editing and positive selection of the B cell repertoire;and 4) to test hypotheses regarding the mechanisms of lineage specificity and allelic exclusion of IgHC V-to-DJ rearrangement. These studies are significant because defective V(D)J recombination can lead to profound immunodeficiency, because mistakes in targeting the recombinase are associated with genomic instability, chromosomal translocations, and malignancy, and because appropriate regulation of the recombinase is necessary to avoid autoimmune disease.